Space-based phased arrays composed of hundreds of elements can be used for microwave communications systems. The phased-array design allows for rapid placement of the beam and rapid adjustment of beam characteristics to avoid jamming. Also, phased arrays degrade gracefully and demonstrate general, flexibility unlike a dish antenna with a fixed beam. Efficiency of the phased array is one important design consideration. Phased arrays usually transmit one signal at any one time. In order to operate in the most efficient manner, the RF amplifiers in the phased array are operated well into compression, which is also near the point of maximum efficiency. Deep compression means that the output power does not change when relatively large changes in input power are made. Phased-array designs often trade amplifier linearity for efficiency as required.
The ability to transmit two or more signals simultaneously through a phased array would directly increase the information carrying capacity of a space-based communication system. However, the transmission of multiple signals could generate in-band intermodulation products. The intermodulation products are due to the amplifiers operating in compression. The amplifiers are operated in compression to keep the efficiency high. Unfortunately, mixing of the signal components within the amplifier when operated in saturation would cause the amplifier output signals to contain large undesirable intermodulation products. These intermodulation products would then produce spurious transmitting beams that could compromise the security of the main transmit channels. The intermodulation distortion in phased-array amplifiers degrades system communications. Intermodulation products can interfere with other desired transmitted signals. Intermodulation products can exceed the specified power level for out-of-band emissions. Intermodulation products can interfere with other users. And, intermodulation products may contribute to interception by unwanted listeners. Reducing the intermodulation products produced within the amplifiers would preserve the signal fidelity, thus making the phased arrays useful in a multiple carrier system.
The conventional nonlinear operation of the amplifiers precludes sending multiple signals simultaneously through them. If this were to be done, large intermodulation products, usually of odd order, such as the 3rd, 5th, and 7th order would be produced. The frequencies of the odd-order intermodulation products are in-band, and would be transmitted to the ground. The transmission of these intermodulation products is likely to exceed constraints on spurious emission levels. Thus, the phased array is usually limited to transmitting a single signal at a time.
The two-tone input signal to an amplifier can be written in the following form: V=sin (ω1t +φ1) +sin (ω2t +φ2). A nominal model of the amplifier transfer function can be written as a polynomial, vout=GV +η1V3+η2V5+η3V7, where vout is the output signal. The inband output signal produced by the amplifier consists of several frequency terms, including the linear terms, sin (ω1t +φ1), sin (ω2t +φ2), the 3rd order intermodulation products, sin ((2ω2−ω1)t +α), sin ((2ω1−ω2)t +β), and the 5th order intermodulation products, sin ((3ω2−2ω1)t +γ), sin ((3ω1−2ω2) t +δ), as well as higher order intermodulation products. The intermodulation product terms decrease in amplitude for each successive order. The most significant intermodulation product term is usually the 3rd order.
Traditional linearization techniques, such as feed forward, digital predistortion, and feedback methods, are routinely applied in order to improve the performance of communication systems that use a single main amplifier. A transmitter designer may select which single or which combination of linearization techniques must be used in order to make possible the transmission of multiple signals. Because the single large amplifier will use most of the DC input power in the system, this amplifier usually sets the overall system efficiency. The extra circuitry needed to implement predistortion or feedback linearization may not significantly degrade the overall efficiency of such a system. The feed forward technique, however, may produce a severe reduction in efficiency. A phased array, on the other hand, contains a large number of amplifiers. If the traditional single-amplifier linearization techniques were applied to the phased array, a large number of linearization circuits would be required. The negative impact on system efficiency, weight, power, and size would render the system impractical.
RF circuits have been developed that specifically cancel intermodulation products that are generated by the interaction of a transmitted signal with an interfering signal introduced at the output port of the transmitter. These RF circuits are typical balanced amplifier designs. In this case, the intermodulation products are generated in the output circuits of the amplifiers, and then reradiated. The intermodulation products that would normally be produced in the amplifiers are reduced by means of an appropriate phase-shifting and combining network at the amplifier outputs. In order to maintain the required output power level of the desired signal, an input splitting and phase-shifting network is also required. This method does not however cancel intermodulation tones generated by the presence of two signals introduced at the amplifier input port. The above described RF circuits can only amplify one signal at a time and still cancel intermodulation products at the output port. These and other disadvantages are solved or reduced using the invention.